This application claims priority to Japanese Patent Application No. 2001-308153 filed on Oct. 4, 2001.
1. Field of the Invention
The present invention relates to an optical near field generator for generating a near field light and methods for generating near field light.
2. Description of the Background
In a conventional optical microscope, light is condensed using a lens. Therefore, resolution is limited by the wavelength of the light. Alternatively, in a near-field optical microscope, light is condensed using a microstructure having a size on the order of nanometers, e.g., an aperture having a diameter of not larger than the wavelength of light, instead of lens. When light is applied to such a microstructure, a localized light called xe2x80x9cnear field lightxe2x80x9d is generated near the microstructure. By approximating this near field light to a sample and allowing it to scan an upper surface of the sample, it is possible to measure the shape and optical characteristics of the sample with a resolution which is determined by the size of the microstructure. Recently, microscopes of this type have been applied to various fields, including measuring the shape and spectroanalysis of biosamples, semiconductor quantum structures, and polymers, as well as in high-density optical recording. xe2x80x9cNear field lightxe2x80x9d as referred to herein means a localized light, i.e., light that has a wave number (k) with an imaginary component.
A widely used optical near field generator (hereinafter referred to as xe2x80x9cnear field optical probexe2x80x9d is a tapered optical fiber (optical fiber probe) having an aperture with a diameter that is not larger than the wavelength of light. The optical fiber probe is fabricated by stretching one end of an optical fiber under heating or tapering it by chemical etching and subsequently coating the optical fiber with metal except at the tip portion of the optical fiber. By introducing light into the optical fiber it is possible to generate a near field light in the vicinity of the aperture formed in the tip of the optical fiber.
However, the above fiber probe is disadvantageous in that the light utilization efficiency is low. For example, when the aperture diameter is 80 nm, the ratio between the intensity of light incident on the optical fiber and that of light output from the fiber tip is 105 or less (See, Applied Physics Letters, Vol. 68, No. 19, pp. 2612-2614, 1996).
In view of this, there has been proposed a probe using a plane metal scatterer. This probe, shown in FIG. 34, includes a plane metal scatterer 341 having a triangular shape formed on a plane substrate. In FIG. 34(a) a probe is shown having one metal scatterer 341, and in FIG. 34(b) a probe is shown having two metal scatterers 341. Upon incidence of X-polarized light, a near field light is localized at a vertex 342. Particularly, by making the wavelength of incident light match the plasmon resonance it is possible to generate a very powerful near field light (See, Technical Digest of 6th international conference on near field optics and related techniques, the Netherlands, Aug. 27-31, 2000, p55). In FIG. 34(a), a near field light is generated from the vertex 342 of the metal scatterer 341, and in FIG. 34(b), the two metal scatterers 341 are disposed such that a vertex-to-vertex spacing is several tens of nanometers or less, with a localized near field light being generated in the vertex-to-vertex spacing 343 (the gap).
The above probes using triangular plane scatterers may attain a higher near field light generation efficiency compared to other methods, especially if the frequency of light and the resonance frequency of plasmon generated in the metal are made coincident with each other. In this conventional example, however, the size and shape of the scatterer are not optimized.
The present invention preferably provides improved shape and size characteristics for a plane scatterer for the efficient generation of plasmon in a probe using the scatterer. A near field optical probe of high resolution and high efficiency may therefore be achieved.
In at least one presently preferred embodiment, a near field light is generated by an electrically conductive scatterer which is tapered toward a near field light generating vertex, and the area of the scatterer is adjusted so as to be smaller than the area of a light spot radiated to the scatterer (herein, the xe2x80x9clight spotxe2x80x9d is a cross-section of the light taken in the plane that is parallel to and at the surface of the scatterer) or smaller than the square of the wavelength of light radiated to the scatterer. Additionally, the distance between the near field light generating vertex of the scatterer and a point which is most remote (i.e., furthest) from said vertex is smaller than the diameter of a light spot radiated to the scatterer or smaller than the wavelength of the radiated light. With this constraint, the phase of the light radiated into the scatterer becomes uniform at various points, and it is thus possible to efficiently excite plasmon resonance.
It is preferable that the area of the light spot radiated to the scatterer be set at a value of not more than one hundred times the area of the scatterer (or, if expressed in terms of length, the light spot diameter on the scatterer be not more than ten times the distance between the near field light generating vertex of the scatterer and a point furthest from the vertex). As a result, it is possible to decrease the amount of light passing without impinging on the scatterer and to improve the light utilization efficiency.
To maximize the energy of light introduced into the scatterer, it is preferable that a central position of light incident on the scatterer be aligned with a central position of the scatterer. Alternatively, in order to maximize a single portion of the near field light, the central position of incident light may be made substantially coincident with the position of the near field light generating vertex. The xe2x80x9csubstantial coincidencexe2x80x9d means that the distance between the central position of the incident light and the vertex is no more than half of the full width at half maximum of the incident light spot. The xe2x80x9cvertexxe2x80x9d as referred to herein indicates not only an actually intersecting point of a first line (side) and a second line (side) but also a point having a predetermined curvature.
In application, the surface of the scatterer and that of a recording medium are preferably set so as to be substantially in parallel with each other. This xe2x80x9csubstantially parallelxe2x80x9d limitation means that the angle between the surface of the scatterer and that of the recording medium be within 5xc2x0. By satisfying this condition, it becomes possible for light to be incident in a direction perpendicular to the surface of a sample or recording medium and hence possible to use an optical system which is employed in a conventional microscope or recording/reproducing apparatus.
The aforesaid angle between the surface of the scatterer and that of a sample or recording medium may also be set at a value in the range of 0xc2x0 to 90xc2x0. In this case, the scatterer is disposed in such a manner that the distance from the near field light generating vertex to the sample or the medium is shorter than the distance from another portion of the scatterer to the sample or the medium. With this arrangement, it is possible to diminish the influence of a near field light generated at an edge portion which lies on the side opposite to the near field light generating vertex.
The above-described scatterer may be formed as a film having, for example, three or more vertices. In this case, for preventing a near field light from being generated at a vertex other than the near field light generating vertex, the radius of curvature of this other vertex is set larger than that of the near field light generating vertex. The scatterer may be in a combined shape of a tapered film and a circular film or in a shape having a curvilinear portion, such as a sectorial shape. By making the radius of curvature of the curvilinear portion larger than that of the near field light generating vertex, it is possible to prevent a near field light from being generated at a portion other than the near field light generating vertex.
The angle of the near field light generating vertex may be changed stepwise. For example, in the aforesaid combined film of a tapered film and a circular film, if a vertical angle (Q in FIG. 4A) at the near field light generating tip is set small and the angle of a portion spaced apart from the tip is set large, the radius of curvature of the circular portion can be set large, and it is possible to decrease the intensity of a near field light generated in the circular portion. Conversely, if the vertical angle (Q) at the near field light generating tip is set large and the angle of the portion spaced apart from the tip is set small, it is possible to decrease the length at the edge portion located on the side opposite to the near field light generating vertex. As a result, it is possible to reduce the total area of the scatterer, and in the case of using many scatterers (as will be described later), it is possible to increase the number of scatterers in the arrangement.
When the angle between the surface of the scatterer and the surface of the sample or recording medium is larger than 0xc2x0 and not larger than 90xc2x0, the angle of the near field light generating vertex may be set at 0xc2x0. This is for the following reason. When the angle of the near field light generating vertex is 0xc2x0, the radius of curvature on the side opposite to the near field light generating vertex becomes small to about the same degree as the near field light generating vertex, and a powerful near field light is generated there. However, there is no such influence if the side opposite to the near field light generating vertex is thus spaced apart from the sample or recording medium.
In the above scatterer, given that the distance between a first tangential line at the near field light generating vertex and a second tangential line on the opposite side parallel thereto is L, a real number part of the dielectric constant of the scatterer is e, the dielectric constant of the surrounding material is em, a coefficient dependent on the material of the scatterer is P, a coefficient dependent on the angle of the near field light generating vertex is A, and a coefficient dependent on the dielectric constant of a medium or sample is M, it is preferable that one of the following conditions be satisfied:
Px(xe2x88x922.5xc3x97(e/em+M+A)+30) less than L less than Px(xe2x88x9220xc3x97(e/em+M+A)+50)
or
Px(xe2x88x9270xc3x97(e/em+M+A)xe2x88x92850) less than L less than Px(xe2x88x9290xc3x97e/em+M+A)+50)
As a result, plasmon resonance can be excited, and it is possible to generate a powerful near field light. The coefficient dependent on the material of the scatterer is set at, for example, 0.5 in a case containing 70% or more of aluminum, 0.8 in a case containing 70% or more of magnesium, 1 in a case containing 70% or more of gold, 1 in a case containing 70% or more of copper, or 1 in a case containing 70% or more of silver. The coefficient M dependent on the dielectric constant of the medium or sample is set at 0 in the absence of any medium or sample, 0 in a case the medium or sample used being a dielectric, or 5 in a case the medium or sample used being a metal or a semiconductor. The coefficient A dependent on the xe2x80x9cQxe2x80x9d angle and is set at A=xe2x88x920.05xc3x97Q+3 where Q stands for the angle of the near field light generating vertex. For generating a powerful near field light, it is preferable that the angle Q of the near field light generating vertex be set at a value in the range of 30xc2x0 to 80xc2x0.
Because the wavelength at which plasmon resonance occurs differs depending on the material, it is preferable that a suitable material be selected for the scatterer in conformity with the wavelength used. For example, if the length L is about 100 nm, it is preferable to use aluminum or magnesium at a wavelength of 300 to 500 nm, silver at a wavelength of 400 to 700 nm, and gold or copper at a wavelength of 500 to 800 nm. It is preferable to choose a material which does not easily undergo oxidation and is not difficult to process. As between silver and gold, gold is the more suitable because it does not undergo oxidation. As between aluminum and magnesium, aluminum is the more suitable because it is the easier to form into a film.
It is preferable that the thickness of the scatterer (in both singular and cases in which a plurality of scatteres are used) be made smaller toward the near field light generating vertex. By so doing, an electric charge gathers toward the near field light generating point, so that it is possible to enhance the near field light intensity.
There may be adopted a construction wherein in the vicinity of a filmy scatterer whose width is smaller toward the near field light generating vertex there is disposed a second scatterer having electric conductivity in such a manner that the closest spacing between the near field light generating vertex of the first scatterer and the second scatterer is not larger than the wavelength of light incident on the scatterers. According to this construction, electric charges present in the scatterers act on each other and a powerful near field light is generated between the scatterers. Particularly, if the second scatterer is also a filmy scatterer whose width is smaller toward the near field light generating vertex, it is possible to generate a very powerful near field light. For the same reason as in case of a single scatterer (above), the total area of the scatterers is preferably set at a value of not larger than the area of a light spot radiated to the scatterers or not larger than the square of the wavelength of light radiated to the scatterers. A longest width portion of the region where the plurality of scatterers are present is set at a value of not larger than the diameter of the light spot radiated to the scatterers or not larger than the wavelength of light radiated to the scatterers. Preferably, the area of the light spot is not larger than one hundred times the total area of the plurality of scatterers, or the diameter of the light spot radiated to the scatterers is not larger than ten times the longest width portion of the region where the plural scatterers are present.
Preferably, the scatterers are two scatterers of the same shape, and one scatterer is oriented in a 90xc2x0 or 180xc2x0 rotated direction of the other scatterer, centered on the near field light generating vertex.
Consideration will now be given to the case where a second scatterer (out of two tapered scatterers) is disposed in a 180xc2x0 rotated direction of a first scatterer, centered on a near field light generating vertex. The material used for the first scatterer and that for the second scatterer are made equal to each other, and the distance L between a first tangential line at the near field light generating vertex and a second tangential line on an opposite side parallel thereto is made equal between both scatterers. In this case, given that a real number part of dielectric constant of each scatterer is e, the dielectric constant of the surrounding material is em, a coefficient P dependent on the material of each scatterer is P, a coefficient dependent on the angle of the near field light generating vertex is A, and a coefficient dependent on the dielectric constant of a medium or sample is M, it is preferable that one of the following conditions be satisfied:
Px(xe2x88x922.5xc3x97(e/em+M+A)xe2x88x9220) less than L less than Px(xe2x88x9220)xc3x97(e/em+M+A)
or
Px(xe2x88x9270xc3x97(e/em+M+A)xe2x88x92900) less than L less than Px(xe2x88x9290)xc3x97(e/em+M+A)
Once this condition is satisfied, it is possible to excite plasmon resonance and generate a powerful near field light. The coefficient P depends on the scatterer material, the coefficient M depends on the dielectric constant of a medium or sample, and the coefficient A depends on angle, and they are set in the same manner as in case of a single scatterer.
The number of scatterers formed in the vicinity of the first scatterer may be two or more. By arranging scatterers in different directions it is possible to increase the number of polarization directions allowable for generating a near field light. Further, by making the scatterers different in plasmon resonance frequency, it is possible to widen the range of wavelength allowable for generating a near field light.
In the case of combining two or more scatterers as mentioned above, it is preferable that a central position of light incident on the scatterers be made substantially coincident with a point at which the total of the distances up to near field light generating vertexes in the scatterers becomes smallest. The xe2x80x9csubstantial coincidencexe2x80x9d means that the distance between the central position of the incident light and each of the vertices is within one half of full width at half maximum of the incident light spot.
If each scatterer is buried in a substrate surface such that a surface of the scatterer to be approximated to a sample or a recording medium and the substrate surface are substantially flush with each other, it is possible to decrease wear of the scatterer. The xe2x80x9csubstantially flushxe2x80x9d means that a difference in height is within 50 nm. Alternatively, in the case where it is necessary to let the probe scan at high speed, such as in an optical write/read of information, a pad portion may be formed in the vicinity of the scatterer in such a manner that the surface of the scatterer for approach to a sample or a recording medium and the pad surface are substantially flush with each other. Also in this case, it is possible to diminish wear of the scatterer, and the xe2x80x9csubstantially flushxe2x80x9d here again indicates that a difference in height is within 50 nm.
A light shielding, or opaque, film may be formed in the vicinity of each scatterer in such a manner that the spacing between the scatterer and the opaque film is smaller than the wavelength of light incident on the scatterer. By so doing, it is possible to prevent the occurrence of background light. For minimizing the occurrence of background light, there may be provided a bonded portion of both the opaque film and the scatterer. This film-scatterer bonding should be done at a portion other than the near field generating vertex and other than the electric charge gathering edge portion on the opposite side. The thickness of the opaque film may be increased for improving the light shielding property or the thickness of the scatterer may be increased for permitting easy access of the scatterer to a sample.
For preventing the occurrence of background light, a second layer having an opaque film may be formed in the vicinity of a first layer where the scatterer is formed. In this case, the first and second layers are spaced a distance not longer than the wavelength of light incident on the scatterer, and an aperture not larger than the wavelength of incident light on the scatterer is formed in the opaque film of the second layer so that the position of the aperture is substantially coincident with a near field light generating vertex of the scatterer. By detecting light which has passed through the aperture in the second layer, it is possible to check a decreased detection of background light.
The scatterer may be formed at a light condensing point on a light condensing element, whereby it becomes unnecessary to make a positional adjustment of the light condensing element and the scatterer. Also, the scatterer may be formed near an exit surface of an optical resonator, more specifically within 10 xcexcm from the exit surface. By so doing, light which has been reflected without impinging on the scatterer is returned by the resonator and is again radiated to the scatterer, so that it may be possible to improve the light utilization efficiency.
The scatterer may also be disposed near an exit surface of a semiconductor laser, more specifically within 10 xcexcm from the exit surface. With this arrangement, it becomes unnecessary to make a positional adjustment of a light source and the scatterer. Further, the scatterer may be formed near a light receiving surface of a photodetector, more specifically within 10 xcexcm from the light receiving surface. This arrangement makes a positional adjustment of the photodetector and the scatterer unnecessary, and it may be possible to diminish the loss of energy generated between the probe and the detector.
The scatterer may be formed on a flat surface at the tip of the projecting portion of a cone or a pyramid. In this case, the area of a portion which approaches a sample or a recording medium decreases, such that it becomes easier to let the probe approach the sample or the recording medium. In this case, moreover, if the side face of the cone or the pyramid is covered with a metallic or light-shielding film and if the area, or a minimum value of width, of the flat surface on which the scatterer is formed is set at a value of not larger than the square of the wavelength of light traveling through the interior of the cone or the pyramid, light can be condensed by the projecting portion of the cone or the pyramid, with a consequent improvement of efficiency.
The scatterer may be formed on a side face of the projecting portion of a pyramid so that the position of a near field light generating vertex and that of the pyramid vertex are substantially coincident with each other. For example, a film which is tapered toward a vertex is formed on one or two opposed faces of a quadrangular pyramid. In this case, since the probe tip is sharp, the measurement of a sample having concave and convex surface portions becomes feasible. The xe2x80x9csubstantial coincidencexe2x80x9d means that the distance between the near field light generating vertex and the pyramid vertex is within 50 nm.
The above probe using the scatterer can be utilized in a near field optical microscope. More particularly, the probe is placed in the vicinity of a sample (within several tens of nanometers in distance), and laser light is radiated to the scatterer. Scattered light and emitted light resulting from interaction between the near field light generated in the probe and the sample are condensed by an objective lens and are detected by a photodetector. By thus using the probe according to the present invention, it becomes possible to measure a sample with both high resolution and high efficiency. It is therefore possible to make a high resolution measurement of a spatial distribution of weak light signals such as emitted light and non-linear light.
The above probe using the scatterer is also applicable to a near field optical recording/reproducing apparatus. For performing a high-speed scan while keeping the spacing between the scatterer and a recording medium at a distance of not larger than several tens of nanometers, the scatterer is formed on a slider, and laser light is directed thereto. Upon incidence of light which is powerful enough to form a bit cell on the recording medium, a bit cell is formed on the medium. In reproduction, light is applied to the scatterer, allowing a near field light to be generated, and scattered light resulting from the interaction between the near field light and the scatterer is condensed by an objective lens and is detected by a photodetector. In this case, as the condenser lens, the objective lens which has been used for introducing light into the probe may be used, or an objective lens disposed on the opposite side with respect to the medium may be used. Thus, by executing recording and reproduction using a probe according to the present invention, it becomes possible to implement a recording/reproducing apparatus which satisfies both high recording density and high transfer speed.
The above probe using the scatterer is further applicable to an exposure system. More specifically, the probe according to the present invention is approximated to an upper surface of a resist formed on a substrate surface, and light is directed thereto, whereby the resist is exposed to a near field light generated by the probe. With the probe according to the present invention, the exposure of a pattern having a size of no more than several tens of nanometers can be done at a high speed.